The present invention relates, in general, to the testing of circuits and, more specifically, to a method and circuit for testing high-frequency mixed-signal circuits with a low-frequency tester.
The number of integrated circuits (ICs) that contain mixed-signal (analog and digital) circuitry is increasing as a percentage of all ICs, and is expected by industry observers to exceed 60% in 2001. The number of digital pins of these ICs is also increasing and can exceed 1000 pins, and the digital data rate applied to the pins is increasing and can exceed 500 MHz. Digital testers that achieve these pin counts and data rates can cost several million dollars. Adding mixed-signal test capability to one of these testers can increase its price by 20% to 40%.
Mixed-signal testers are significantly more expensive than digital testers, primarily for three reasons: (1) a high signal-to-noise ratio (SNR) must be achieved in the presence of many high speed digital signals; (2) the analog stimulus frequencies must be a precise fraction of the digital clock frequencies; and (3) the analog circuitry of the tester must operate at higher speed and accuracy than the circuits to be tested.
The two most common mixed-signal circuits are analog-to-digital converters (ADCs) and digital-to-analog converters (DACs). The accuracy and speed of converters has increased over the years, and can exceed 16 bits at 100 MHz. To test converters, a tester typically needs to be faster and more accurate than the converter being tested, but it is becoming more difficult and expensive for testers to have this performance advantage because tester performance and IC performance are converging as fundamental limits to circuit technology are approached. When testing an ADC or DAC within a larger circuit, an N-bit digital output or input is accessed by the tester as N digital signals in parallel, or serially at a rate equal to N times the sample rate. N is typically between 6 and 16. Testing high frequency (sample rate) DACs and ADCs at their maximum operating speed (xe2x80x9cat-speedxe2x80x9d) is necessary because their performance typically degrades as operating frequency increases.
It is nevertheless possible to test a DAC or ADC at a clock rate that is much less than the normal operating speed of the ADC or DAC. For example, differential non-linearity (DNL), the maximum difference between the actual voltage step sizes and the ideal step size of a DAC, is commonly measured by providing a digital input value to the DAC; measuring the resulting output voltage, V1, for a period long enough to filter out noise; provide a second digital value, higher by one bit, to the DAC; measuring the filtered output voltage, V2; calculating V2-V1; and comparing the calculated value to an ideal value.
Using this sequence, all output voltage differences are measured. However, the DAC is tested at a rate slower than its maximum speed because time must be allowed for a sufficiently accurate measurement of its low-pass filtered output voltage, and, because the output analog voltage increments by only one least significant bit-equivalent at a time, this is much slower than the largest increments that must be converted, which can be more than one half of the 2N step full-range.
U.S. Pat. No. 5,825,786, granted to M. Burns on Oct. 20, 1998, for xe2x80x9cUndersampling Digital Testability Circuit,xe2x80x9d describes a circuit for testing the integrity of high data rate transfers by undersampling the digital input to a DAC being tested, but does not otherwise test the DAC itself. The patent does not teach how to analyze the analog signal output of the DAC to detect defects. Undersampling refers to the technique of sampling a signal at a sample rate that is slower than twice the highest frequency of interest in the sampled signal.
Burns U.S. Pat. Nos. 5,578,935 granted on Nov. 26, 1996, for xe2x80x9cUndersampling Digitizer with a Sampling Circuit Positioned on an Integrated Circuitxe2x80x9d and 5,589,763 granted on Dec. 31, 1996, for xe2x80x9cCoherent Undersampling Digitizer for Use with Automatic Field Test Equipmentxe2x80x9d, are concerned with the difficulties associated with accurate measurement of high frequency signals using Automated Test Equipment caused by stray capacitance and inductance from the fixturing such as wafer probes, handler contactors and device sockets may have a detrimental effect on the high frequency electrical signals to be measured. Exemplary, the settling time of a palette digital to analog converter (DAC) could be difficult to accurately measure through a long wire since the inductance of this long wire would adversely affect the signal to be measured. The ""935 patent proposes incorporating a comparator on the same integrated circuit chip as the circuit under test so that stray capacitance and inductance is minimized if not eliminated. The ""763 patent proposes the use of a coherent undersampling digitizer for use with automatic field test equipment. Both patents describe a method for deducing the value of a periodic signal at any instant in its cycle by repeatedly comparing the signal to a reference voltage that successively approximates the signal based on the result of the comparison. The patents do not teach how to generate a high-frequency stimulus for the CUT without using a high frequency tester. Further, the methods require an algorithmic search for each sample""s value, which can produce erroneous results in the presence of high frequency noise.
A paper, entitled xe2x80x9cAnalog DFT Using an Undersampling Techniquexe2x80x9d published in 1999 in by R. Mason proposes the use of undersampling to allow the observation of high frequency signals via a low frequency on-chip analog bus. An analog switch samples a signal of interest and uses an analog bus as a sample-and-hold capacitor. The sampling switch is controlled by pulses, having a duration, t, and a period, T, under control of a sampling clock, and is used to observe signals with a bandwidth less than 0.5/t, according to Nyquist theory, while conveying signals on the analog bus at frequencies less than 0.5/T. In the paper, 1 MHz sampling clocks having pulses between 2 and 0.5 ns duration were used to observe signals with between 180 MHz and 1.1 GHz bandwidth. The analog bus signal was conveyed off-chip at frequencies below 500 kHz by an analog buffer. The sampling switch used in this paper is a conventional complementary metal oxide semiconductor (CMOS) transmission gate. Mason does not address the fundamental problem of how to generate a high frequency stimulus for the circuit under test. In addition, the technique requires two high frequency signals, the applied signal and a sampling clock whose frequency is only xe2x80x9cslightly offset from the sinusoid signalxe2x80x9d. For high frequencies, it can be expensive to generate two such frequencies, for example, a 200 MHz sinusoid and a 199.99 MHz square wave, accurately and with low jitter. Further, the technique requires an on-chip hold capacitor, which limits the amount of low pass filtering that can be accomplished with this capacitor to reduce the high frequency noise.
The present invention seeks to reduce the cost of mixed-signal testers and to simplify high frequency mixed-signal testing by using low frequency stimulus signals; generating and sampling analog signals at a constant low frequency, independently of digital sampling signals used during testing; and facilitating serial data access at rates that are much less than the sampling rate. The present invention also seeks to implement the circuit in a manner that minimizes extra circuitry for circuits constructed according to the IEEE 1149.4 standard.
The term xe2x80x9chigh frequencyxe2x80x9d herein refers to the sampling rate or the frequency of the signal input to the device under test. The term xe2x80x9clow frequencyxe2x80x9d refers to the frequencies of signals used to generate the high frequency signal applied to the device under test as well as to the data access rate of the output of the device.
In accordance with the present invention, at least two low frequency signals, each being either an analog signal or a digitally encoded version of an analog signal, are multiplexed using a high frequency clock to produce the required high frequency input stimulus signal which is applied to a circuit under test (CUT). One or more of the low frequency signals can be an unchanging signal (frequency equal to zero). A high frequency output of the CUT is sampled synchronously to the high frequency clock but at a lower frequency. The samples are stored temporarily. Low frequency properties of the signal samples, linearity, for example, are measured to determine properties of the high frequency output of the CUT. It will be seen, therefore, that a tester need not be equipped to generate the high frequency stimulus signal because the high frequency stimulus signal is easily generated from low frequency signals well within the capability of existing mixed signal testers. Further, the tester need not have the capability of processing the results at high speed. The results can be processed at a rate corresponding to the frequency of the low frequency input signals.
The sampling circuitry is preferably located within the same integrated circuit as the CUT. The sampling times and the sampling pulse width are preferably adjustable. The effect of the multiplexer or sampling switches on the signals is minimized by using a modified T-switch, in which the gates of each leg of the T are controlled separately. The multiplexer or sampling switches can be subsumed into an 1149.4 analog boundary module, and controlled by a common clock. If the sampled signals are analog voltages, the capacitance of the storage capacitance can be increased to reduce noise, by connecting an off-chip capacitance in parallel, and signal degradation across the capacitance due to leakage current can be reduced by connecting a current source whose value is equal to the measured leakage current.
One aspect of the present invention is generally defined as a method for testing an analog, or mixed analog and digital, circuit that operates at a clock frequency, the method comprising: multiplexing a plurality of low frequency stimulus signals under control of a high frequency clock to produce a high frequency input stimulus signal; applying the input stimulus signal to a circuit under test so as to obtain a circuit output signal; sampling the circuit output signal synchronously with the high frequency clock at a frequency equal to the clock frequency divided by the number of the plurality of low frequency stimulus signals to produce a sequence of samples; storing said samples; and measuring properties of said samples to determine properties of the circuit.
Another aspect of the present invention is generally defined as a circuit for sampling an analog waveform, the circuit comprising a sampling switch for sampling the analog waveform; a sample storage capacitance including a single capacitance and a plurality of capacitances selectably connected in parallel; a sampling pulse generating circuit for generating pulses having a duration significantly less than one half of the period of a sampling frequency.
A further aspect of the present invention is defined as a circuit for testing an analog, or mixed analog and digital, circuit that operates at a clock frequency, the circuit comprising a multiplexer for multiplexing a plurality of low frequency stimulus signals; multiplexer control means for producing a multiplexer select signal under control of a high frequency clock to cause the multiplexer to sequentially select each of the low frequency stimulus signals and produce a high frequency stimulus signal for application to a circuit under test; sampling means for sampling an output signal of the circuit under test in synchronism with the high frequency clock and at a frequency equal to the clock frequency divided by the number of the plurality of low frequency stimulus signals; means for storing samples produced by said sampling circuit; and means for measuring properties of said samples to determine properties of said circuit under test.